The present invention relates to a rotor for a disc brake, particularly though not exclusively, for applications in the car field.
It is known that the rotors of the above-mentioned type consist of two parts sharing a rotation axis. A first part, the support hub, is destined to be connected to the wheel hub of a vehicle, whereas the remaining peripheral part, the braking band, is destined to cooperate with the disk brake calipers to carry out the braking action on the vehicle. The braking band can be either solid or, such as in the examples represented in the annexed figures, ventilated. The ventilated band comprises two binders being connected by a plurality of bridges, such as to define cooling air ducts.
With reference to the rotor and the rotation axis thereof, any direction parallel to said axis is defined as being axial, any perpendicular direction to the axis and incident thereto is defined as being radial, and any direction tangential to a circumference centered on said axis and laying on a normal plane to said axis is defined as being tangential.
It is also known that, when the brake is being operated, friction between the brake caliper pads and the braking band surface generates a considerable amount of heat must be released. The generated heat causes a number of unwanted phenomena, including overheating of the components and the consequent thermal expansion and distortion of the same.
In the disc brake field, the need is particularly felt to make the braking band of a material which, besides ensuring the desired friction characteristics, is capable of maintaining its mechanical characteristics as unchanged as possible upon increase in the operating temperature. In view of the above, it is particularly convenient to manufacture the braking band of cast iron.
Likewise, the need is particularly felt of manufacturing the support hub of a material that is as light as possible, to reduce the mass of the rotor and consequently the non-suspended masses of the vehicle. In view of the above, it is particularly convenient to manufacture the support hub of a light alloy, such as aluminium alloy.
With reference to the prior art rotors, it should be noted that the distortions caused by heat strain entail inconveniences in the operation of the disc brake, especially uneven wear of the brake caliper pads. This is mainly due to the fact that, contrary to the braking band, which tends to radially deform while remaining coplanar with itself, the support hub is deformed by taking the shape of a cone that also defines the twist of the braking band. The deformation of the support hub substantially defines the twist and accordingly the loss of coplanarity of the braking band.
From what has been stated above, there emerges a double need of maintaining unchanged the transmission of the braking torque from the braking band to the support hub and, at the same time, allowing the braking band to dilate radially, due to the heat stress to which it is subjected as much independently from the support hub as possible.
The patent EP1092889, in the name of the same applicant, describes a rotor in which the hub of aluminium alloy and the braking band of cast iron are joined by a plurality of stainless steel pins. The pins, which are interference coupled in the braking band and slackly in the hub, make it possible for the band to radially dilate. This solution is particularly expensive and complicated from the point of view of construction. Furthermore, the small section of the steel pins induces high stress peaks in the material in the step of transmission of the braking torque from the band to the hub.
It is also known from U.S. Pat. No. 6,152,270, in the name of the same applicant, a rotor having a braking band of cast iron and a support hub made of a light alloy. Both pieces are produced by subsequent castings in such a way as to be intimately adhered, so that they are integral to each other. This solution, though being cost-effective and easy to implement, does not allow the band to radially dilate independently from the hub.